The present invention relates to a method for reversibly storing hydrogen using alkali metals or their hydrides and aluminum metal as hydrogen storage materials and doping with transition metal catalysts.
According to the patent application PCT/WO 97/03919 of the Studiengesellschaft Kohle mbH (SGK), a method for reversibly storing hydrogen is known using the alkali metal alanates of general formula M1p(1xe2x88x92x)M2pxAlH3+p (1) as storage materials, where M1=Na, K; M2=Li, K; 0xe2x89xa6xxe2x89xa6xcx9c0.8; 1xe2x89xa6pxe2x89xa63. To improve the hydrogenation/dehydrogenation kinetics, the alkali metal alanates are doped with transition metal compounds in catalytic amounts. In particular, the alanates NaAlH4, Na3AlH6 and Na2LiAlH6 are employed.
The drawbacks of the previous SGK method are that the preparation and purification of commercial sodium alanate, the preparation of Na3AlH6 or Na2LiAlH6 and the subsequent doping in organic solvents are relatively complicated on a preparative level and in most cases require the use of solvents which are highly volatile and highly inflammable (ether, pentane) or tend to form peroxides (ether, THF).
Surprisingly, it has now been found that instead of using the transition-metal doped alkali metal alanates as hydrogen storage materials, the starting materials used for their preparation in the form of alkali metal hydrides or alkali metals (especially NaH and Na), Al powder and doping agents can be employed. The alanates formed in one hydrogenation step from such starting materials are immediately functioning as H2 storage materials and have improved storage properties as compared to PCT/WO 97/03919.
Methods for the preparation of alkali metal alanates from alkali metal hydrides (or alkali metals), aluminum and hydrogen are known. A survey of the methods for the preparation of NaAlH4, Na3AlH6 and Na2LiAlH6 can be found in J. Alloys and Compounds, 298 (2000) 125-134. Thus, according to the German Patent Specification 1 136 987 (1962), Na and Li alanates can be prepared by reacting the corresponding alkali metal hydrides (or alkali metals) and aluminum in ethers, amines and aliphatic or aromatic hydrocarbons, optionally in the presence of catalytic amounts of organoaluminum compounds, with hydrogen under pressure. The U.S. Pat. No. 3,138,433 (1964) describes, inter alia, a method for the preparation of NaAlH4 from NaH, Al and hydrogen under pressure in THF using Ti, Zr, Hf and Th tetrahalides as catalysts; however, in the only patent example contained therein, a maximum of 21.8% is stated as the yield of NaAlH4. The direct synthesis of Na3AlH6 can be successfully performed with 98% yield according to Inorg. Chem. 5 (1966) 1615 by reacting Na and activated Al powder in diglyme in the presence of Et3Al with hydrogen under pressure (350 bar). The synthesis of NaAlH4 from the elements Na, Al and H2 is also possible in the absence of organic solvents according to Dokl. Akad. Nauk SSSR 215 (1974) 1369, Engl. 256, by performing the process in the molten state (xe2x89xa7175 bar,  less than 280xc2x0 C.). The alkali metal alanates prepared according to the methods mentioned were not considered for purposes of hydrogen storage.
In contrast, the preparation of the storage material according to the present invention is very simple, completely dispensing with organic solvents. The aluminum powder used according to the present method is cheaper and more easily handled than sodium alanate, which was previously employed. It was particularly surprising that the hydrogenation of aluminum in the presence of alkali metals or metal hydrides can be successfully performed at temperatures which are considerably below the melting points of the metal/metal hydride educts involved and the metal alanate products, i.e., in a solid state (in contrast to the above referenced direct synthesis according to Dymova et al., Dokl. Akad. Nauk SSSR 215 (1974) 1369, Engl. 256 xe2x80x9cDirect Synthesis of Alkali Metal Aluminium Hydrides in the Meltxe2x80x9d).
According to the present invention, for example, aluminum powder is mixed with powdered sodium hydride and admixed with catalytic amounts of titanium tetrabutylate. The composition thus obtained can be used directly as a reversible hydrogen storage material, When Al and NaH are employed at a molar ratio 1:1, NaAlH4 is obtained in the hydrogenation, whereas a molar ratio of 1:3 yields Na3AlH6 after hydrogenation.
Another particular advantage of the present method for reversibly storing hydrogen is that the desorption and absorption kinetics could be significantly improved by facilitating the previously known method according to PCT/WO 97/03919.